The present invention relates to a method for detecting and responding to a short-circuit occurring in a circuit having a plurality of power semiconductor devices of a turn-off type connected in series after detection of the short-circuit and to devices for accommodating such situations.
Such circuits, having power semiconductor devices connected in series, may be used in voltage-stiff converters in stations of plants for transmitting electric power through High Voltage Direct Current (HVDC), or for reactive power compensation (RPC), for converting direct voltage to alternating voltage and conversely. These converters may typically have to maintain voltage within the range 10-500 kV, although other voltages are conceivable. This makes it possible to connect numerous such power semiconductor devices in series to distribute the voltage among them since they normally each may only hold 1-5 kV. However, the invention is not restricted to such so-called high voltage converter circuits, even though such a special application will be described hereinafter for illustrating the problem upon which the present invention is based.
Examples of such power semiconductor devices of turn-off type are turn-off thyristors (GTO), MOSFETs and IGBTs (Insulated Gate Bipolar Transistors). The latter are preferable in many respects, since they combine good power handling ability with properties which make them well suited for connection in series in so-called IGBT valves in converters, for example, since they may be turned on and turned off simultaneously with high accuracy.
Short-circuit situations may, in rare cases, occur in these circuits, and it is then necessary to be able to handle these in such a way that entire IGBT valves do not break down. Such short-circuits may in such an application arise, for example, through fault of the control apparatus controlling the power semiconductor devices, which may mean that the so-called DC capacitor normally located on the direct voltage side of said station is discharged so that a short-circuit current will flow through one of the phase legs of the converter. A phase leg is formed by two IGBT valves in the same direction. Another possible short-circuit is between a phase terminal and one of the terminals of the DC capacitor or between two phase terminals, or that a ground fault occurs in a phase terminal and the DC capacitor is formed by two capacitors having a grounded midpoint.
If a short-circuit occurs in a phase leg provided with IGBT valves, the following happens: The individual IGBTs take such a voltage that the short-circuit current is limited to a value 3-10 times higher than the nominal current through the IGBT valves under normal operation. The IGBTs make this due to a current-limiting IV characteristic thereof, just as do other transistors, During the current-limiting process, the power semiconductor devices are forced to very high power dissipation. This may only exist only during a very restricted period of time, typically 10 .mu.s.
Thus, when a short-circuit occurs, it is desirable to be able to turn off the different power semiconductor devices as soon as possible to prevent the breakdown of any of them. However, there is a problem because of variations in the drive units controlling each individual power semiconductor device, which means different fast turn-off. Also, individual variations between the different power semiconductor devices results in that they take different voltage during the current-limiting process and are turned off differently. This as well as other individual variations therebetween, results in considerable problems during the current-limiting process as well as at turn-off when a short-circuit occurs.
During both the current-limiting process and the turn-off process, the entire series connection of the power semiconductor devices has to take a certain voltage, in which the voltage to be taken by a certain power semiconductor device may through such variations be too high, where it may be considerably lower and completely acceptable across other power semiconductor devices. This means a high risk of failure of any power semiconductor device, this may, for example, happen when the voltage across a power semiconductor device gets so high that the high field strength generated thereacross leads to "avalanche", meaning that nearly all the power is concentrated at a certain point. It is also possible that the high voltage, in combination with a high short-circuit current, may lead to an overheated power semiconductor device breaking down as a consequence.
The present invention relates, according to a second aspect, to a method for detecting a short-circuit in a circuit having a plurality of power semiconductor devices of turn-off type connected in series, in which the voltage across the two electrodes of each individual power semiconductor device is measured and compared with a reference voltage value which is higher than the maximum voltage level across the electrodes at a given instant at normal operation of the power semiconductor device. Also, a short-circuit may be detected when the voltage of the power semiconductor device in question exceeds the reference voltage value. A device for enabling such a detection is also disclosed.
In such described methods, according to prior art, the on-state voltage across the power semiconductor device is detected indirectly by detecting the occurrence of a current through a diode when a power semiconductor device is turned on, in which the diode has the anode thereof connected to a low voltage (typically +15V) and the cathode is connected to the collector of the power semiconductor device. The current through the diode is expected when turned on at a time when the voltage across the electrodes of the power semiconductor device has decreased under the low voltage and the detection of a short-circuit occurs when the expected current does not occur. However, this means that the short-circuit is detected after a long delay in some situations, with the result that the voltage across the power semiconductor device in question has had time to increase too much and, at the same time the current therethrough has had time to reach a very high value before a turn-off procedure is started. Thus, there is a risk that the power semiconductor device will be destroyed as a consequence of "avalanche" or too high power dissipation therein.